The present invention relates to optical measurement instruments and more particularly to an improved optical sensor for such instruments having an electrically adjusted mosaic filter.
Optical measurement instruments, such as photometers, radiometers and the like, are used to measure spatial and spectral qualities of radiation in the ultraviolet to infrared portion of the electromagnetic spectrum being emitted, reflected or transmitted by an object. Most measurements of radiation in this spectral region fall into one of two categories, radiation received by a surface and radiation emitted or scattered by a surface. Photometers measure radiation in the visible light spectrum and use the human eye or its equivalent as the sensor. Radiometers measure radiation from the ultraviolet through the infrared spectrum and are designed to have a nearly equal response to as wide of range of wavelengths as possible.
Two basic designs are used in photometers, one where the human eye is the sensor and the other where a photosensitive device, such as a vacuum photocell, photoconductive cell, silicon photodiode or the like, is used. The human eye has a spectral sensitivity that responds differently to light of different colors and has a maximum sensitivity in the yellow/green region. Through experimentation, the relative radiant sensitivity of the human eye to specific wavelengths of light has been determined and a photopic curve has been defined by the Commission Internationale de L'Eclariage (CIE) and universally accepted as the standard for the spectral sensitivity of the average human eye.
The photosensitive devices used in photometers have spectral responses that do not match that of the average human as defined by the CIE photopic curve. To use these devices in photometers, optical filters are placed in front of the devices to selectively absorb the wavelengths of light to correct the sensor's response to that of the average human eye. There are two basic type of optical filters used in optical sensors for photometers, a homogeneous filter or subtractive filter and a mosaic or additive filter. The homogeneous filter is constructed out of two or more pieces of absorbing filter glasses cemented together. The pieces of glass used are uniform in transmittance across their surfaces. Radiation incident on the filter is modified by passing, in turn, through each of the spectrally selective filter glasses, which remove parts of the incident radiation spectrum by absorption. The mosaic filter is constructed using a homogeneous glass or glasses that conform somewhat to the CIE photopic curve. Smaller pieces of glass filters are cemented to the surface of the homogeneous glass to decease the transmittance in specific spectral regions. By adjusting the position and size of the smaller glass filter pieces, the filter's transmittance can be made to conform more accurately to the photopic curve. Accuracy of the photopic correction is a basic advantage of the mosaic filter over the homogeneous filter.
Having to adjust the size and position of the various glass filter pieces requires hand assembly by an experienced technician and is a major drawback to mosaic filters. Additionally, as the various pieces of filter glass are added, it is necessary to re-measure the optical sensor filter-detector combination against the CIE photopic curve.
What is needed is an optical sensor for an optical measurement instrument that does not require the selective sizing and placement of glass filter elements in order to match the sensor to an specific spectral response. Such an optical sensor should be easy to assemble without having to continually re-measure the sensor against the desired spectral response as the glass filter elements are added.